1. Field of the Invention
The present invention relates to a developing apparatus for use in a copying machine, a printer, a facsimile machine or other machines that use an electrophotography scheme, electrostatic recording scheme or the like.
2. Description of the Related Art
The following method has been generally used in image forming apparatus that forms color images using, for example, an electrophotography scheme. That is, toner images of plural colors are sequentially formed on an electrophotographic photosensitive member (photosensitive member) as an image bearing member, so that the toner images of plural colors are superposed on a transferring medium such as a recording sheet sequentially or at one time.
In the past, in that type of image forming apparatus for forming color images, a so-called rotary developing method has been proposed and put to practical use. The rotary developing method is such a method in which a rotor mounted with developing units for respective colors such as black, yellow, magenta and cyan is rotated so as to sequentially bring the developing units to be used to a position opposed to a photosensitive member, at which developing operations are performed.
On the other hand, in conventional image forming apparatus, especially in image forming apparatus that form color images, a two-component developing method that uses a mixture comprised principally of non-magnetic toner and magnetic carrier as a two-component developer has been widely used.
The two-component developing method has advantages with respect to stability in image quality and durability of apparatus as compared to other developing methods presently proposed. However, long term use of the image forming apparatus of the two-component developing method inevitably involves deterioration of the developer and especially deterioration of the carrier, so that replacement of the developer is required, which has invited an increase in service costs and operating costs.
Some measures for solving the above problem have been proposed. Especially, a so-called trickle developing method has been put to practical use, which replenishes the developing device in the rotary developing method, with the two-component developer utilizing changes in the direction of the gravitational force involved by the rotation of the rotor and also discharges the two-component developer from the developing device utilizing the same principle.
For example, Japanese Patent Application Laid-open No. 9-218575 discloses a rotary developing apparatus as shown in annexed FIG. 11 of the present application, in which a developing device 101 is replenished with developer including an amount of toner for supplementing the toner consumed in the developing operation at the developing position P1 at which the developing device 101 is opposed to a photosensitive member 128, and surplus toner in the developing device 101 is discharged therefrom and recovered by a developer cartridge 105 through a recovering port 111, a discharge pipe 112 and a inlet port 110, utilizing changes in the gravitation direction involved by the rotation of a rotor 118. Such a rotary developing apparatus has a simple structure and prevents the charging ability of the carrier from deteriorating without bringing about a significant increase in the size of the image forming apparatus or increase in its manufacturing cost, since the apparatus makes use of an inherent motion of the rotor to replenish and recover the developer.
On the other hand, Japanese Patent Application Laid-open No. 10-142888 discloses a developing apparatus as shown in annexed FIG. 12 of the present application, in which developer discharged from a developing container 101 is temporarily stored in storing portion 113 at a developing position P1 at which the developing device 101 is opposed to a photosensitive member 128, then transferred to a cylindrical shaft 118a disposed at the center of a rotor 118 utilizing the changes in the gravitation direction involved by the rotation of the rotor 118, and finally recovered by a developer recovering container, which is provided at an end of the cylindrical shaft 118a, by means of a developer transferring member 120 provided in the cylindrical shaft 118a. As per the above, this developing apparatus also discharges the developer by making use of an inherent motion of the rotor, like the above-described rotary developing apparatus proposed by Japanese Patent Application Laid-open No. 9-218575. Therefore, this apparatus also prevents the charging ability of the carrier from deteriorating, without bringing about an increase in the size of the image forming apparatus. In addition, with the construction of this developing apparatus, even in the case in which monochromatic (or mono-color) image formations are successively performed, surplus toner in the developing device is discharged, at the developing position, to the storing portion outside the developing device without interrupting the developing operation, so that the developing apparatus can keep the amount of the developer in the developing device within a permissible range without bringing about a decrease in image productivity.
Recently, with increases in trade of full color copying machines and full color printers, demand for a variety of functions has been placed on them. Along with this, many image forming apparatus that are pursuing high image productivity while focusing on downsizing and cost-reduction have been manufactured as commercialized products, and such apparatus are expected to be a part of the mainstream in the future.
The rotary developing apparatus described in the above-mentioned Japanese Patent Application Laid-open No. 9-218575 utilizes the rotational motion of the rotor to discharge deteriorated developer to the developer cartridge, so that its structure is simple, and that it does not involve an increase in the size of the image forming apparatus or an increase in cost. In addition, in this rotary developing apparatus, since the properties of the developer as a whole are stabilized by the gradual replacement of the deteriorated developer with new developer, exchange of the developer is not required, so that ease of maintenance is enhanced. However, in this rotary developing apparatus, since surplus developer in the developing device is not discharged to the exterior of the developing device at the developing position, when high density monochromatic images are formed successively, it is necessary, in order to prevent an increase in the developer in the developing device, to stop the developing operation to perform the developer discharging operation, i.e. to perform rotation of the rotor, even though the image formation is still carrying on. So a decrease in image productivity was inevitable. Generally speaking, it is the case that users of color image forming apparatus use them for monochromatic black image forming more often than color image forming.
On the other hand, in the developing apparatus described in the above-described Japanese Patent Application Laid-open No. 10-142888, deteriorated developer is discharged to the recovering container provided at an end of the rotor shaft so as to gradually replace the deteriorated developer with new developer, thereby stabilize the properties of the developer as a whole. Therefore, exchange of the developer is not required, so that ease of maintenance is enhanced. Furthermore, in this developing apparatus, since surplus toner in the developing device is discharged, at the developing position, to the storing portion outside the developing device, image productivity will not be lowered by the developer discharging operation. However, in this developing apparatus, since the developer stored in the storing portion is transported to the interior of the rotor shaft first and then further transported to the recovering container by means of the developer transporting member provided in the shaft, the developing apparatus needs to be equipped with various parts, the recovering container, and a driving device, which are not necessary for normal image forming operations, which has inevitably increased the complexity of the image forming apparatus and significantly increased the manufacturing cost of the apparatus.
To summarize the above, in the prior art technologies, it is true that replenishment of the developing device with not only toner but also with the carrier eliminates the need for the exchange of the deteriorated carrier as a whole so as to enhance ease of maintenance, but in the prior art arrangements, in the case of a printing job in which a plurality of monochromatic images (especially high density images) are successively formed, the image forming operation must be frequently interrupted in order to rotate the developing rotor (for the purpose of only discharging surplus developer), since in the prior arts, the surplus developer is discharged to the exterior of the developing device utilizing the rotation of the developing rotary. Therefore, it has been impossible to meet two requirements, that is, ease of maintenance and high image productivity, while attaining simplification of apparatus construction and cost reduction.